can someone please tell me what my shihtzu is mixed with??

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hello i have 2 female shihtzus and now i have zack(male)..i was told he was pure breed..i love him to pieces but really want to know what he is mixed with..his tail is not curled and his snout is longer and his hair isnt growing like my girls did..please can someone help me determine what his other breed could be..he is 4 mths old now and is 9 lbs...he is my baby boy but i need to know..thankyou for any input